Studying planets as worlds and compare them with each other is called comparative planetology

Planetology is applied to any noticeably large object in the system (planets, moons, asteroids, comets)

To start we need to seek clues to the origin of the Solar system 3Four Challenges

Pattern of Motion

All planets orbit the Sun in the same direction (counterclockwise as seen from the Earths North Pole)

Planet orbits are nearly circular and co-planar

Planets rotate in the same direction which they orbit

Almost all moons orbit their planets in the direction of the planet rotation

The Sun rotates in the direction planets orbit it

Explain Why is this order so good? 4Four Challenges

Different types of planets

Two distinct groups of planets

Terrestrial planets (Mercury, Venus, Earth, Mars)

Small, rocky, abundant in metals, few moons

Jovian planets (Jupiter, Saturn, Uranus, Neptune) Large, gaseous (made of hydrogen and its compounds), no solid surfaces, have rings, a lot of moons (made of low-density ices and rocks)Explain Why is the inner and outer Solar system divided so neatly? 5Four Challenges3. Asteroids and CometsAsteroids are small, rocky bodies that orbit the Sun mostly between Mars and Jupiter (the asteroid belt) Almost 9,000 asteroids have been discovered Comets are small and icy bodies that spend most of their lives beyond the orbit of Pluto They occupy 2 regions Kuiper belt and Oort cloudExplain The existence and general properties of the large number of these small bodies 6 Four Challenges4. Exception to the RulesMercury and Pluto have larger orbital eccentricities Uranus and Pluto have tilted rotational axes Venus rotates backwards (clockwise) Earth has a large moon Pluto has a moon almost as big as itselfAllow for these exceptions 7The Nebular TheoryThe Solar system was formed from a giant, swirling interstellar cloud of gas and dust The hypothesis was originally suggested by Immanuel Kant (1755) and Pierre-Simon Laplas (1790)A cloud is called nebula - nebular hypothesisThe collapsed piece of cloud that formed our own solar system is called the solar nebula 8Collapse of the Solar Nebula

Three important processes gave form to our system, when it collapsed to a diameter of 200 A.U.

The temperature increased as it collapsed

The rotation rate increased

The nebula flattened into a disk (protoplanetary disk)

9Evolution of the Solar System 10Building the PlanetsInitial composition 98 hydrogen and helium, and 2 heavier elements (carbon, nitrogen, oxygen, silicon, iron)Condensation the formation of solid or liquid particles from a cloud of gas Different kinds of planets and satellites were formed out of different condensates 11Ingredients of the Solar SystemMetals iron, nickel, aluminum, etc. Condense into solid form at 1000 1600 K 0.2 of the solar nebulas massRocks primarily silicon-based minerals Condense at 500 1300 K, 0.4 of the massHydrogen compounds methane (CH4), ammonia (HN3), water (H2O) Condense into ices below 150 K, 1.4 of the massLight gases hydrogen and helium Never condense in solar nebula 98 of the mass 12Condensation 13AccretionAccretion is growing by colliding and sticking The growing objects formed by accretion planetesimals (pieces of planets)Small planetesimals came in a variety of shapes, reflected in many small asteroids Large planetesimals (gt100 km across) became spherical due to the force of gravityInner solar system only rocks and metals condensed and only small bodies formed 14Nebular CaptureNebular capture growth of icy planetesimals by capturing larger amounts of hydrogen and helium It led to the formation of the Jovian planetsNumerous moons were formed by the same processes that formed the protoplanetary disk Condensation and accretion created mini solar systems around each Jovian planet 15The Solar WindSolar wind is a flow of charged particles ejected by the Sun in all directions It was stronger when the Sun was youngThe wind swept out a lot of remaining gas and interrupted the cooling of the nebula If the wind were weak, the ices could have condensed in the inner solar system 16Leftover PlanetesimalsPlanetesimals remained from the clearing became comets and asteroids They were tugged by the strong gravity of the jovian planets and got more elliptical orbits Rocky leftovers became asteroids Icy leftovers became comets 17Planetary Evolution - GeologicalInternal heating leads to geological activity volcanism, tectonicsAs core cools and solidifies, activity slows, and eventually stops (Moon)Earth and Venus are large enough to be active 18Planet Activity 19Planetary Evolution - Atmosphere

Atmospheres are formed by

gases escaping from interior

- impacts of comets (volatile-rich debris)

Fate of water depends on temperature (distance from the Sun)Atmospheres changed chemically over timeLife on Earth substantially changed the atmosphere 20Other Planetary SystemsOver 100 extrasolar planets have been discovered since 1995 The Extrasolar Planet EncyclopediaStars are too far away from the Sun, and direct imaging cannot detect planets near them Current strategy involves watching for the small gravitational tag the planet exerts on its star The tag can be detected using the Doppler effect 21Extrasolar Planets in the Sky 22Planet Transits 23The Nature of Extrasolar PlanetsThe discovery of extrasolar planets gives us an opportunity to test the solar system formation theory Most of the discovered planets are different from those of our system They are mostly Jupiter-size and located closer to their stars But possible planet migration discovered planets are exceptions 24SummaryAll the planets were formed from the same cloud of dust and gas Chance events may have played a large role in the formation and evolution of individual planets Planet-forming processes are apparently universal

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